This is a great point. The mailing list and public nature of the standards process makes it very difficult to subvert, without very high risks of getting caught and breaking trust in the community. These agencies need to keep hiring good cryptographers and ideally keep bodies working on standards.
Shows the importance of OSS in security and having people like Trevor Perrin keeping watch.
But at the same time - if the NSA was going to subvert encryption standards - I doubt they would subvert the process with someone who is known to work at the NSA. Intelligence agencies would operate covertly. Most likely by converting someone trusted in the community into an agent, or grooming their own agent straight out of high school/university and getting them to a point of influence in the community (over a long period of time) and only then having them damage crypto standards. < this is standard tradecraft.